One type of hammer drill employs a driven annular ratchet which by operator pressure can be forced against a fixed annular ratchet causing the driven ratchet to be moved axially until the teeth disengage and the housing moves forward to deliver an impact to the drill bit. The magnitude of the impact is a function of the pressure the operator applies to the tool. The hammer action can be locked out so the tool functions as a drill only.
A self tapping screw has been introduced for use in concrete and the like. Removal of such screws is difficult and is preferably done by a tool. This suggests a reversible hammer drill but if the motor is reversed with the hammer drill just described with the ratchet not locked out the ratchet can lock up and deliver full torque to the operator, likely breaking his wrist. With such potential it is scant comfort that locking the ratchet (hammer) out of action can avoid the problem. Therefore, there is need for a positive arrangement for preventing driving engagement of the ratchet during reverse operation.